Communication devices such as wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a wireless communications system or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications system.
Wireless devices may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as wireless device or a server.
The wireless communications system covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the area of radio coverage provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in Global System for Mobile communications (GSM), may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP GSM EDGE Radio Access Network (GERAN), a wireless device has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. EDGE is an abbreviation for Enhanced Data rates for GSM Evolution. In the end of 4008 the first release, Release 8, of the 3GPP Long Term Evolution (LTE) standard was finalized and later releases have also been finalized.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
During the last few years cellular operators have started to offer mobile broadband based on WCDMA/High-Speed Packet Access (HSPA). Further, fuelled by new devices designed for data applications the end user performance requirements are steadily increasing. The large uptake of mobile broadband has resulted in heavy traffic volumes that need to be handled by the HSPA networks. Therefore, techniques that allow cellular operators to manage their spectrum resources more efficiently are desirable.
To improve the DL performance techniques such that 4-branch Multiple-Input Multiple-Output (MIMO), multiflow communication, multi-carrier deployment may be introduced. Since improvements in spectral efficiency per link are approaching theoretical limits, the next generation technology is about improving the spectral efficiency per unit area. In other words, the additional features for High Speed Downlink Packet Access (HSDPA) may need to provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks. Currently, 3GPP has been working on this aspect of using Heterogeneous networks, as discussed in RP-121436, Study on UMTS Heterogeneous Networks, R1-124512, Initial considerations on Heterogeneous Networks for UMTS, Ericsson, ST-Ericsson, and R1-124513, Heterogeneous Network Deployment Scenarios, Ericsson, ST-Ericsson.
Homogeneous Networks:
A homogeneous network is a network of base stations, such as Node B, in a planned layout and a collection of user terminals in which all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations offer unrestricted access to user terminals in the network, and serve roughly the same number of user terminals. Current cellular wireless system comes under this category for example GSM, WCDMA, HSDPA, LTE, Wimax, etc.
Heterogeneous Networks:
In heterogeneous networks, in addition to the planned or regular placement of macro base stations, several pico/femto/relay base stations are deployed as shown in FIG. 1. FIG. 1 illustrates a typical deployment of low power nodes, or small cells 1, in a Heterogeneous Network with a macro node 2.
The power transmitted by these pico/femto/relay base stations may be relatively small compared to that of macro base stations which may be up to 2 Watts (W) as compared to that of 40 W for a macro base station. These Low Power Nodes (LPN) are deployed to eliminate coverage holes in the homogeneous networks using macro only, improving the capacity in hot-spots. Due to their lower transmit power and smaller physical size, pico/femto/relay base stations may offer flexible site acquisitions,
The Low power nodes in heterogeneous networks may have
a. Different cell identifier as that of macro cell, that is, different cells;
b. Same cell identifier as that of macro cell, called soft, shared, or combined cell.
Combined Cell in a Heterogeneous Network
As mentioned above, heterogeneous networks may be divided into two categories
1. Where the low power nodes have different cell identifiers (ids) than that of the macro node.
2. Whew low power nodes have same cell id as that of the macro node.
FIG. 2 shows a heterogeneous network layout with two low power nodes, a macro node. In the network of FIG. 2, the low power nodes create different cells, Cell B and Cell C, and each of the low power nodes has different cell id. The macro node creates Cell A. In the Figure, the logical cell coverage is marked with dashed and continuous lines. Simulations show that using low power nodes in a macro cell offers load balancing, with large gains in system throughout as well as cell edge user throughput.
One disadvantage of the above method is that each LPN creates a different cell, hence a UE may need to do soft handover when moving from one LPN to a macro or to another LPN. The increase in higher layer signaling which is needed to perform handovers may have a negative impact on network performance. With a large number of LPN, mobility management may become worse.
FIG. 3 shows the heterogeneous network where low power nodes are part of the macro cell. The individual cells created by each of the low power nodes and the macro node form a single cell, or the same cell, also called soft cell A. In the figure, the Common Pilot CHannel (CPICH), sometimes also referred to as P-CPICH, is represented with a dashed line. This is sometimes called a soft cell or a shared cell. This set up avoids the frequent soft handovers and, hence, lowers the requirements on higher layer signaling. The drawback with this shared cell deployment is that the capacity of the system is limited since the LPN and macro site covers the same area.
FIG. 4 shows the typical configuration of a combined cell deployment where the central controller in the combined cell takes responsibility for collecting operational statistics information of network environment measurements. The decision of which nodes to transmit to a specific UE is made by the central controller based on the information provided by the UEs, network nodes or on its own. The cooperation among various nodes may be instructed by the central controller and may be implemented in a centralized way.